Radio frequency coupling system



Feb. 5, 1935. 1.. BABIK ET AL 1,989,730

RADIO FREQUENCY COUPLING SYSTEM Filed May 14, 1952 3 Sheets-Sheet l WM N & lw 2 I 10 I I I I 5 INVENTORS LUDWIG BABIK ANDREAS JAUMANN Feb. 5, 1935. BABIK ET AL 1,989,730

RADIO FREQUENCY COUPLING SYSTEM Filed May 14, 1932 5 Sheets-Sheet 2 I k i N 11 5 INVENTORE LUDWIG BABIK ANDREAS JAUMANN ATTORNEY Feb. 5, 1935. L. BABIK ET AL 1,989,730

RADIO FREQUENCY COUPLING SYSTEM Filed May 14,1932 3 Sheets-Sheet 3 ATTORNEY fulfill a number of important requirements. The

Patented Feb. 5, 1935 RADIO FREQUENCY COUPLING SYSTEM Ludwig Babik, Berlin-Reinickendorf, and Andreas Jaumann, Berlin-Charlottenburg, Germany, assignors to Siemens & Halske, Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Application May 14, 1932, Serial No. 611,268 In Germany April 30, 1931 4 Claims. (01. 178-44) The object of this invention is a radio-frequency amplifier which primarily is to be used for the measurement of low radio-frequency potentials. For this purpose the amplifier should insure such low strays and winding capacities as have been found by theoretical calculations. Also the choke-coil coupling which, by the use of two chokes one of which corresponds to the main (series) inductance, and the other one to the leakage "inductance of the transformer, can theoretically be made equivalent to the transformer coupling, fails to fulfill the demands laid down. Also the use of special damping in the choke coupling does not give the desired result since in order to secure uniform amplification the damping would have to be chosen so high that a considerable and inadmissible decrease in gain would be the inevitable result.

Now, according to the present invention the above requirements are satisfied by means of a circuit scheme which represents the combination, of a capacity-resistance coupling and the coupling by means of a tuned transformer. Instead of the transformer coupling recourse could be had also to a choke-coil coupling which, apart from the voltage-transformation by the transformer, could be made equal to the same by the useof two chokes simultaneously.

In accordance with the basic idea of the invention the capacity-resistance coupling means and the tuned transformer or inductances equivalent thereto are so connected and dimensioned that the transmitting properties of the coupling chain system of this invention inside the range of long Waves (down to, say, 600 meters) are determined essentially by the resistance elements of the system, while inside the adjacent transmission range, especially at the upper limit thereof, conditions are largely governed by the reactance means of the system or chain. In other words, the transmission: or transfer as it were, is divided between the resistance and the reactance elements. In so far as the transfer properties of the latter are concerned the position of the main resonance and the stray resonance are decisive. The former is determined by the main inductance and the tuning and tube capacitances in parallel relation thereto, whereas for the secondary resonance the leakage inductance and the capacities between the electrodes of the tubes are decisive. The dimensions to be chosen for the coupling means are based upon this consideration that the amplification, for main and secondary resonance, has approximately the same value as the gain insured by the resistance means for the lower frequencies. The frequency corresponding to the main resonance is chosen Within or near the frequency range where the chief desideratum is practically perfect constancy of amplification inside a Wide wave-length range, say, for a coverage extending from 200 to 3000 m. Furthermore, by the use of cascade arrangements the amplification is to be pushed very high. In addition, of course, all chances of self-excitation (oscillation) should be precluded.

In broadcast reception work almost exclusively amplifiers comprising several tuned circuits are used for radio-frequency amplification. However, schemes of this kind are unserviceable for the present purpose inasmuch as they fail to insure adequate stability or constancyin gain inside a wave-length range as above indicated. The further demand of uni-control which is the only one adapted to insure in schemes of the said sort a well-defined gain and calibration is attended with expensive construction.

Known in the prior art is also what has been called an aperiodic radio-frequency amplifier in which the provision of special resonant structures is dispensed with. However, also with these circuit schemes it is impossible to measure up to the requirement of sufiicient gain constancy. Fig. 1 is a graph showing the voltage-amplification of a two-stage aperiodic radio-frequency amplifier as a function of the wave-length. Although in this instance one of the well-known models of two-stage amplifiers is concerned, the graph evidences an obvious decrease in gain with growth of frequency. It will be noted that this amplifier would fulfill the requirements only inside a range extending from 1000 to 3000 meters.

Experiments made with transformer coupling also failed to give satisfactory results. The transformer was first used in the last stage, i. e., for coupling a radio-frequency amplifier tube with the rectifier. Although this case was particularly favorable because it is only the constant capacity of the rectifier tube that plays a part, the amplification evidences a very irregular shape as can be seen from Fig. 2, and conditions would be still furtherimpaired if the transformer coupling were used between the tubes of the lower stages, as then the dynamic tube capacity would play a part. Another point is that the problem of self-oscillation of the amplifier becomes more important. Insufiicient usefulness of the transformer coupling for the present purpose inheres primarily in that it is practically not possible to partial gain secured by the resistance means alone begins to fall off appreciably.

By the use of damping means it is possible in connection with such choosing of dimensions to obtain a .very extensive uniformity of the gain; and the damping may be insured both by paralleling of distinct resistances in reference to the inductances as well as by suitable proportioning of the self-damping (self-resistance, iron core). The invention can be carried into effect by the use of air-cored or iron-cored coils; by the use of the latter type the construction becomes more compact. I

According to another object of the invention the properties of the amplifier are improved lbyv that its lower liminal frequency is shiftedinto a In this:

region above the tone-frequency range. Way disturbing noises, especially those due to the ringing of the tubes, can be obviated. The preclusion of tonal frequencies (audio-frequencyl from the tube can be effected by the suitable dimensioning of the coupling condenser and the grid leak, the latter being, if desired, dimensioned at the same time in such a way that it will insure adequate damping of the leakage reactance.

Figs. 3 and 4 illustrate graphically measuring results obtained with amplifiers designed in accordance with the invention. The curves 1-4, Fig.3 refer to 1-, 2-, 3-, and 4-tube amplifiers. The coupling inductances used therein consisted of iron-cored coils. The graph in Fig. 4 shows the amplification of a two-stage amplifier utilizing air-cored inductances. I Capacity-resistance amplifier arrangements which, in order to obtain a certain frequency de- I pendence, are provided with means for adding inductances have been known in the prior art in difierent forms in audio-frequency amplification. In this instance the inductances serve primarily for the correction of distortions below or above the amplifier. In other words, they will distort what would nearly be a rectilinear amplification characteristic of the capacity-resistance amplifier in the audio-frequency range, if dimensions have been chosen properly. Inside the audio-frequency range it is comparatively easy to insure a definite shape of amplification. The theoretically calculated values for the coupling means are practically realizablewithout any serious difficulties so that the amplifier will exhibit the desired frequency dependence. But in radio-frequency amplification the situation becomes essentially more difficult. Indeed, it has so far not been possible to build a radio-frequency amplifier which presents properties like those of the amplifier of this invention. The combined resistance and choke-coil coupling represents a circuit scheme which makes it possible to secure by the aid of practically realizable inductances, that is, inductance which will always inhere leake age inductances, and winding (turn) capacitance of a definite size, practically constant gain or amplification inside a frequency band of an order of 'magnitude as indicated at the outset. The

inductive coupling means heretofore employed for radio-frequency amplifiers made it necessary to; choose such proportions, in order to obtain constant gain, that due to the distributed selfcapacities entirely uncontrollable and incalculable resonances are apt to arise, and these are likely'to impair or vitiate the. straightnessin the amplification curve. In fact, it is only by means of a circuit arrangement of the kind here disclosed that the coupling means may be made of such proportions that thesaid drawbacks will be eliminated.

Figs. 5 and 6 illustrate two embodiments of the fundamental idea of this invention. In the case of Fig. 5 the coupling chain between the two amplifier tubes R1 and R2 comprises a combination of resistance elements and tuned transformer or coil, the latter (being indicated at Tr) being in series connection with the plate resistance Ra. The primary of the transformer is tuned by the aid of condenser C1. The secondary of the transformer is in coupling relation by way of the condenser K with the grid of the tube R2. Between the gridand the filament of this tube is inserted a grid leak By. The grid-plate capacitance Cga, the grid-filament capacitance Cola and the plate-filament capacitance Cale are indicated by broken lines. The size thereof, just like that of the condenser Cl, is decisive for the position of the resonance points and frequencies.

In Fig-'6 the transformer "is replaced by an equivalent choke-coil arrangement. The chokecoil L1 corresponds to the main inductance and the choke-coil L2 to the leakage inductance of the transformer. Themain inductance being in series with the plate resistance Ra. is tuned by the agency of condenser Cl, while for the tuning of the leakage inductance the inter-electrode capacitances are decisive. It will be understood that inc'hoosing' the dimensiona the self-capacities of the coil must be taken into account. While no damping resistances are shown in the drawings theymay be utilized inside the scope and spirit of this invention with a View toimprove the shape of the amplification curve.

The coupling arrangement according to this invention is useful for'single-stage as well as for multi-stage amplifiers with equal success. For a great number of measuring purposes using the radio-frequency amplifier scheme above a bandpass 'filter may be recommendable.

We claim:] i

- 1. In combination with a pair of cascaded radio frequency amplifier tubes, a coupling network connecting the output electrodes of the first tube to the input electrodes of the second tube, said network including a fixed inductive reactance connected between said output electrodes, a condenser connecting one side of the reactance to the signal control electrode of the second tube,'a resistor connecting the said input'electrodes, said condenser and resistor being so proportioned, with respect to the inherent inter-electrode capacities of the second tuba-that radio frequency energy is substantially uniformly transferred through said network between arange of substantially 600 to 3000 meters, said reactance having a. condenser across it and being tuned to a wave length adjacent the lower end of said range, said reactance and condenser being so chosen that said uniform energy transfer range is extended to transferred through said network between a range of substantially 600 to 3000 meters, said reactance having a condenser across it and being tuned to a wave length adjacent the lower end of said range, the reactance and condenser being so chosen that said uniform energy transfer range is extended to substantially 200 meters.

3. In combination with a pair of cascaded radio frequency amplifier tubes, a coupling network connecting the output electrodes of the first tube to the input electrodes of the second tube, said network including an inductive reactance connected between said output electrodes, a condenser connecting one side of the reactance to the signal control electrode of the second tube, a resistor connecting the said input electrodes, said condenser and resistor being so proportioned, with respect to the inherent inter-electrode capacities of the second tube, that radio frequency energy is substantially uniformly transferred through said network between a range of substantially 600 to 3000 meters, said reactance being tuned to a wave length adjacent the lower end of said range and so chosen that said uniform energy transfer range is extended to substantially 200 meters, and said reactance consisting of a transformer having its primary shunted by a tuning condenser.

4. In combination with a pair of cascaded radio frequency amplifier tubes, a coupling network connecting the output electrodes of the first tube to the input electrodes of the second tube, said network including an inductive reactance connected between said output electrodes, a condenser connecting one side of the reactance to the signal control electrode of the second tube, a resistor connecting the said input electrodes, said condenser and resistor being so proportioned, with respect to the inherent inter-electrode capacities of the second tube, that radio frequency energy is substantially uniformly transferred through said network between a range of substantially 600 to 3000 meters, said reactance being tuned to a wave length adjacent the lower end of said range and so chosen that said uniform energy transfer range is extended to substantially 200 meters and said reactance consisting of a choke coil shunted by a tuning condenser.

LUDWIG BABIK. ANDREAS JAUMANN. 

